Because my helper can't speak Chinese and can only speak English with my girl.
Me and dad speak Chinese. We speak English to her when we read her books written in English.
I have lived overseas many years but still insist my girl must know how to communicate Chinese.
In general, always better to communicate with your best language. It's worse to end up speaking wrong or broke English.
Singapore tries to enforce English but not many people can speak it well, you know.
Also Chinese is still a very important language and much harder to learn than English. If your child has no exposure to Chinese, more difficult to learn Chinese when they are older. Especially important if your child is intending to stay in HK or go to local schools. Then must know Chinese.